Dyes are commonly used and preferred as colorants in ink jet inks because of their high chroma, brightness, and transparency. Dyes, however, present numerous print durability disadvantages, particularly in the water-based inks dominantly employed in consumer and commercial ink jet printers. Such dyes are typically water-soluble and consequently exhibit poor print waterfastness and poor bleed control when printed next to other colors or subsequently subjected to humid conditions. Humid bleed normally produces hue shifts and decreased print sharpness.
Dye color is also highly dependent on media chemistry (e.g., pH). A dye-based ink printed on one print media will take on a different hue when printed on a different print media of the same color. It is therefore difficult to control critical colors, such as facial tones, over a wide media set.
Moreover, dyes typically have poor lightfastness and fade at different rates, depending on color. In many applications, such as digital photography, archival stability is critically important. Commonly-used dyes, such as H-acid azo magenta dyes, can lose 20% of their optical density within three months of indoor light exposure. By comparison, common silver halide photographs maintain this level of optical density for 15 to 30 years. Dye life in inks is shortened still further by photocatalytic reaction between mixtures of different dye colorants. For example, an H-acid azo magenta dye fades within a few weeks under indoor light exposure when printed in contact with a copper phthalocyanine cyan dye. Dye fade is also impacted by humidity, ions, and airborne pollutants. Absorbed moisture has a profound effect and is believed to deaggregate dye in print films, allowing greater exposure to oxygen, while simultaneously increasing the level of oxygen present. Fade rate differences between dyes used in a common print cause annoying color shifts in familiar objects such as facial tones (e.g., shift to green or red).
Color stabilizing additives, such as antioxidants and free radical scavengers, are well-known and have been widely used in colored inks and coatings to improve dye lightfastness. Such additives, however, typically have little impact on ink jet ink printed on common porous substrates, such as paper and media containing porous print surfaces (used to aid print dry times). It is believed that the dye and additive are separated chromatographically as the ink penetrates through the media pores. In general, many additives are dye-specific and may photocatalyze inappropriately matched dyes. For this reason, it is difficult to add such photostabilizing additives to media surfaces coatings.
Certain metal complex dyes have shown significant improvement in dye lightfastness. Some H-acid azo magenta dyes complexed with copper, for example, exhibit 10 to 20 year print lightfastness under indoor exposure conditions. These copper dyes show good lightfastness across a broad media set. Metal complex dyes, however, have low chroma and brilliance. It is believed that the metal promotes dye aggregation, which is known to reduce color properties. Heavy metal dyes also impose toxicological concerns for home use. Metal dyes have the same humidity, color bleed, and waterfast problems associated with metal-free dyes.
Laminants and print overcoatings have also been widely used to protect dye-based prints from photo and moisture degradation. Typically, a transparent plastic film (e.g., Mylar®) is laminated over the print surface. The film provides a significant oxygen and moisture barrier to the underlying print and the dye contained therein. Polymer overcoatings provide the same barrier properties. Laminants, in particular, provide photo parity or better in color and print durability for both indoor and outdoor exposure. Unfortunately, the addition of a lamination or coating station to a printer is inappropriate in cost and footprint for most home and office-use printers. Such addition also reduces print throughput and adds unwanted cost to each print. Laminants and print overcoats are still further disadvantageous because they do not provide a plain paper document solution.
More recently, the ink jet ink industry has employed dispersed pigment colorants in place of dyes. The pigments are typically 100 to 150 nm particles comprising heavily aggregated, normally crystalline dye or metal oxides. As such, pigments typically have greater waterfastness, humid bleed resistance, and media-independent color properties over conventional dyes. Moreover, pigments exhibit far greater photo-stability than conventional dyes. Some pigmented inks show lightfastness of 50 or more years prior to losing 20% optical density. As with aggregated colorants in general, however, pigments exhibit a noticeable reduction in chroma and brilliance over conventional dyes. Pigments also introduce gloss non-uniformities in prints, since the optical and structural properties of the pigments vary from color to color. Gloss non-uniformity is particularly unacceptable for photographic prints. Like conventional dyes, pigments suffer from photocatalytic fade with other colorants. Pigments also have relatively poor transparency, such that overprinted colors tend to mask underprinted colors.
Heretofore, there is no colorant solution that provides both the color properties (chroma, brilliance, transparency) and durability (waterfastness, lightfastness, humid bleedfastness) required in the office and photo ink jet printer markets. Colorants that provide good chroma and brilliance have poor durability and visa versa.
Recently, there is growing interest in UV-curable inks for ink jet applications. UV-curable inks allow higher print throughput and ink durability through the elimination or reduction of water and through ink binder crosslinking, respectively. Many applications, such a textile printing, are targeted, applications that conventionally use high chroma dyes. Only pigments have been successfully used as colorants for UV-curable inks, however, because photoexcited oxygen and initiator-produced free radicals fade dye colorants.
By contrast to inks, dyes are commonly used as colorants for plastics. Dye-colored transparent plastic is common to many light-stable products, including instrument windows, automotive taillights, bubble lenses for light emitting diodes, tableware (cups, plates, eating utensils), candy wrappers, and the like. The polymers in these plastics encapsulate the dissolved dye, greatly restricting the permeation of oxygen to the dye. Without oxygen, the photo-oxidation process predominantly responsible for dye fade does not take place.
Attempts to solve one or more of the foregoing problems have been disclosed.
U.S. Pat. No. 5,484,475, entitled “Micellar-Based Ink Compositions” and issued to M. P. Breton et al on Jan. 16, 1996, discloses the use of dye-micelle chemistry to enhance ink dry time when printed. The micelle is loosely formed around individual dye molecules, which are water-soluble, by an ethoxylated alcohol. However, since the micelles are only associated with, but not bonded to, the dye, then the micelles offer virtually no protection to the dye, and typically dissociated when printed.
U.S. Pat. No. 5,942,560, entitled “Colored Resin Fine Particle Water Base Dispersion Liquid for Water Base Ink” and issued to H. Idogawa et al on Aug. 24, 1999, discloses water-based ink compositions that are stated as being waterfast and lightfast. The colored resin fine particle water-base dispersion liquid is produced by mixing a water-soluble basic dye with a mixed vinyl monomer containing a vinyl monomer having an acid functional group and emulsion-polymerizing the mixture. However, the acid-dye salt coupling is water-soluble, as is the basic dye, and hence subject to dissociation. Some fraction of dye is thus certain to bleed out of the particle during particle formation and after printing.
U.S. Pat. No. 5,990,202, entitled “Dual Encapsulation Technique for Preparing Ink-Jet Inks” and issued to K. C. Nguyen on Nov. 23, 1999, and assigned to the same assignee as the present invention, discloses an ink-jet ink including a vehicle and a colorant, the colorant encapsulated by or associated with a primer core/shell polymer to form a primer/colorant combination, and the primer/colorant combination, upon printing on a print medium, is encapsulated by a durable core/shell polymer. That patent is well-suited for its intended purposes. However, while the encapsulant is polymerized, the colorant itself is not polymerized.
U.S. Pat. No. 5,998,501, entitled “Process for Producing Aqueous Ink for Inkjet Printing” and issued to T. Tsutsumi et al on Dec. 7, 1999, discloses a process of producing an aqueous ink for ink jet printing. The process comprises dissolving a salt-forming group-having polymer and a hydrophobic dye in a water-insoluble organic solvent to obtain a solution, adding water and a neutralizing agent optionally together with a surfactant to the solution to ionize the salt-forming group of the polymer, emulsifying the resulting mixture, and removing out the solvent from the emulsion to obtain an ink containing an aqueous dispersion of the polymer particles in which the dye has been encompassed. However, the process involves several complicating steps that include expensive cleanup and solvent extraction processes.
There remains a need for stabilizing ink-jet inks against the adverse effects of oxygen using a process that is relatively simple. Further, any polymer employed as part of a stabilizing system must be in a form that limits any polymer-induced viscosity increase that is invariably associated with polymer additions to ink-jet inks. While such a form is attained with particulated polymers, there are thermal shear issues in thermal ink-jet printing that can cause such particles to agglomerate and stick to surfaces. Thus, developing a stabilization system must address these various concerns.